1. Field of the Invention
The present invention related to improvement of a power converter for converting a direct current to an alternating current or an alternating current to a direct current, and in particular to a construction technology suitable for size reduction of a power converter.
2. Description of the Related Art
Hitherto, an inverter referred to as a 3-level inverter is known as a kind of converter. Besides the high-potential point and low-potential point, a d.c. voltage source used for this 3-level inverter has an intermediate-potential point between the high-potential point and the low-potential point. This type of inverter is configured to select one potential level from three levels of potential at the high-potential point, low-potential point and intermediate-potential point by turning on and off of included switching devices, and to provide the selected potential at an a.c. terminal. Therefore, the 3-level inverter may raise the PWM switching frequency apparently, and then provide a.c. output with less harmonic content.
This type of 3-level inverter is disclosed in (1) JP-B-51-47848, (2) JP-B-53-14744, (3) JP-A-56-74088, (4) JP-A-56-115182, (5) JP-A-56-121374, (6) JP-A-57-177284, (7) JP-A-63-502953, (8) JP-A-64-34182, and (9) JP-A-1-198280, for example.
Inverters having relatively large capacities such as variable voltage variable frequency (VVVF) inverters are often arranged with switching arms for each phase in a linear arrangement relation.
In the above described conventional converter apparatus, however, wiring lengths from voltage dividing capacitors to switching devices of respective phases are long and unequal. The wiring inductance may cause excessive voltages in snubber circuits of respective switching devices or a certain device, resulting in problems of excessive voltage in snubber circuit of the switching devices, increase of snubber loss, and requirement of size increase in snubber circuits. Especially in large-capacity apparatuses, therefore, it is keenly demanded to make wiring short and uniform to the utmost.
An arrangement of d.c. filter capacitors is disclosed in JP-A-57-206279. The arrangement is a three-phase full-bridge inverter comprising switching devices in six arms, wherein a d.c. filter capacitor is disposed for each phase, thereby the wiring length is shortened between the d.c. filter capacitor and switching device of each phase to the utmost, resulting in reducing loss in the snubber circuits and in obtaining small size of the apparatus.
However, this prior art relates to a so-called 2-level inverter having only two levels in output phase voltage as the same levels as at the high-potential point and the low-potential point of the d.c. power source. And the capacitors thereof separately disposed for respective phases are used only for filtration.